The description from the home page:
"weinre is a debugger for web pages, like FireBug (for FireFox) and Web Inspector (for WebKit-based browsers), except it's designed to work remotely, and in particular, to allow you debug web pages on a mobile device such as a phone."
Isn't that great? You can have the developer tools you are used to back in your greedy little hands again.
The weinre home page has detailed instructions on how to setup your own server, which is not that difficult, but if you are like me (i.e. lazy) there is already a server setup for you at debug.phonegap.com where getting started is only three steps away.